This paper reports the design and performance of the NASA/Grumman heat pipe radiator experiment which was successfully flown on Shuttle flight STS-8. It was the first zero-g demonstration of the high-capacity monogroove heat pipe concept which NASA is planning to use in its space constructible radiator system for future large space stations. The subscale 1.85-m-long U-shaped test article used Freon-21 working fluid and was configured with a double-sided radiator for heat rejection and electrical heaters for heat input. It was operated in the Shuttle payload bay environment for over 2 hours under a 70-watt sustained load. A unique feature of this experiment was the use of temperature-sensitive liquid crystal film to monitor thermal response by astronaut observation of color changes.